"Superantigens" are known or postulated substances that are potent, polyclonal T cell activators in the presence of cells bearing MHC class II molecules. The known superantigens include a variety of exoproteins from Staphylococcus, Streptococcus, and Mycoplasma species, as well as antigens controlled by the Mls system in mice. Each superantigen appears to preferentially stimulate T cells bearing antigen receptors (TcR) representing one (or a few) particular Vbeta region(s). In addition, the T cell response is not restricted by a particular class II allele or isotype. Superantigens appear to act by binding to class II molecules and then interacting with non-polymorphic regions of the TcR. Microbial toxins that act as superantigens are implicated in infectious gastroenteritis, toxic shock syndrome, rheumatic fever, and chronic arthritis in rodents. A potential role for superantigens in the development or exacerbation of chronic autoimmune diseases has been proposed. The staphylococcal superantigens include the enterotoxins (SEA, SEB, SEC1,2,3,SED,SEE) and the toxic shock syndrome toxin (TSST). In general, these proteins bind to class II molecules with high specificity and affinity. For individual toxins, however, there can be marked variation in the ability to bind to certain class II isotypes of alleles. This variation can be exploited to study the binding of given toxins with class II molecules. The goal of this project will be to obtain a detailed understanding of the molecular events responsible for the interaction between the staphylococcal superantigens and MHC class II molecules. In order to achieve this goal the following series of experiments will be undertaken: 1. Map the complete extent of the SEA/SEE binding site on HLA- DR1, 2. Transfer the SEA/SEE binding site to a human class I molecule to test whether T cell activation is dependent only on toxin binding, or requires participation of other class II-specific structural elements, 3. Create and test mutants of murine class II to facilitate mapping the binding site of TSST, 4. Map the binding site for SEB on human class II molecules, 5. Test toxin derivatives for immunomodulatory activity. Defined mutations will be made in the alpha and beta chains of human or mouse class II molecules using recombinant DNA methodologies. These mutant class II molecules will be transfected into fibroblasts and tested for the ability to bind a variety of staphylococcal toxins using a microcytofluorimetric assay. In addition, synthetic toxin derivatives will be created by recombinant DNA techniques and expression in bacterial systems. These novel toxins will be tested for the ability to block authentic toxin action as well as their effects on class II-restricted antigen presentation.